June 25, 2013

Study Uncovers Evolutionary Formula For Large Body Size In Mammals

A team of international scientists has found a connection between mammals’ body size and evolutionary development, according to a new report in the journal Proceedings of the Royal Society B.

The study examined the maximum size of mammalian species – including whales, primates and rodents – since the last ice age and found the weight of a baby mammal relative to its adult body mass is a key factor in determining whether a mammal will evolve into a larger form.

If a species grew more quickly and produced relatively larger young each year, they tended to evolve to a larger maximum size, the study said. They are also likely to reach maximum size in fewer generations, the researchers added.

"Size is fundamental to your life and your body – how fast your heart beats, how much food you need to eat, and how you move," explained study co-author Alistair Evans of Monash University's School of Biology.

"The blue whale is the largest animal to have evolved, even larger than dinosaurs, and it reached this size at the fastest rates we recorded," said Evans. “Key to this success is that they produce large young that mature quickly, reaching around (98 feet) in eight to ten years.”

The research team noted this rate of biological production is crucial, despite the number of offspring produced each year. Lead author Jordan Okie from Arizona State University said primates sizes, for instance, evolved much differently than whales.
"Primates have a low production rate and have evolved very slowly," he said. “They have never got bigger than about (1100 pounds).”

The researchers also made a connection between maximum size and mortality rate. Because larger mammals tend to breed less frequently, the researchers predicted their maximum body size to be about 16 times smaller if the mortality rate doubled.

"This is a really surprising finding," Evans said. "It points to why many of the large animals went extinct after the last Ice Age, as changing climates probably increase mortality rates.

“Large animals are also at high risk of extinction in modern environments because it takes a long time for their population to rebound from disasters,” he added.

In their conclusion, the research team noted their findings support the notion body size and evolution are linked.
“This work highlights the intimate interplay between the macroecological and macroevolutionary dynamics underlying the generation and maintenance of morphological diversity,” they wrote.

The researchers said their future work plans to include determining how extinction risk may be reduced in the face of climate change. Many reports have pegged a marsupial, Australia’s white possum, as the first species driven to extinction by climate change.

White possums have been known to thrive in the cool climes of high-altitude rainforest, but rising temperatures in the animal’s native habitat of Tropical North Queensland have caused some observers to claim it is extinct because of its inability to properly regulate its body temperature. Despite several intense searches for the animal, it has not been seen for years.

Many experts say several of Australia’s insects and frogs have already become extinct via the effects of global warming and many other species are expected to become endangered by rising temperatures.